Provide OCaml env vars for ocamlrep crate (#9635)

Summary:
build.rs in the ocamlrep crate expects a working OPAM env, so provide it in the setup file.

Pull Request resolved: #9635

Reviewed By: paulbiss, Wilfred

Differential Revision: D83145448

fbshipit-source-id: 6a8849e296cb7c306ec92be375c75124e306a525

Author: mszabo-wikia

hphp/hack/dev_env_rust_only.sh.in

@@ -11,4 +11,6 @@
 #  . $BUILD_DIR/hphp/hack/dev_env_rust_only.sh
 
 . "@CMAKE_CURRENT_BINARY_DIR@/dev_env_common.sh"
-# Nothing else to do for rust :)
+
+# Export OCaml variables, needed by ocamlrep
+eval $(opam env)